Vertical reference system

ABSTRACT

A VERTICAL REFERENCE SYSTEM IS PROVIDED FOR SUPPLYING OUTPUT ELECTRICAL SIGNAL PROPORTIONAL TO THE SINES OF THE ANGLES OF TILT OF A STRUCTURE WITH RESPECT TO THE VERTIAL ABOUT TWO TILT AXES AT RIGHT ANGLES TO EACH OTHER. THE SPECIFIC SYSTEM DISCLOSED INCLUDES A VERTICAL GYROSCOPE HAVING A ROTOR MOUNTED IN INNER AND OUTER GIMBALS ON A GYROSCOPE SUPORT WHICH IN TURN IS RIGIDLY SECURED TO THE STRUCTURE SUBJECT TO BEING TILTED. TWO FORCES BALANCED ACCELEROMETERS ARE SECURED TO THE GYROSCOPE SUPPORT AND EACH IS POSITIONED TO SENSE THE ANGLE OF TILT OF THE STRUCTURE ABOUT ONE OF SUCH TILT AXES. THE ACCELEROMETERS ARE EMPLOYED IN CONJUNCTION WITH PICKOFFS AND TORQUES ON THE AXES OF THE GIMBALS TO MAINTAIN THE SPIN AXES OF THE GYROSCOPE VERTICAL. THE OUTPUT SIGNALS ARE OBTAINED FROM THE PICKOFFS ON THE GIMBAL AXES.

United States Patent [72] Inventor James W. Deer Portland, Oreg. 2| 1Appl. No. 776,717 [22) Filed Nov. 18, 1968 [45] Patented June 28, 1971[73 I Assignee Electronic Specialty Company,

Portland, Oreg.

[54] VERTICAL REFERENCE SYSTEM 5 Claims, 5 Drawing Figs.

52 US. Cl 74/56, 73/504, 74/5.8 [51] Int. Cl .r G0lc19/28, G0 1p 9/02[50] Field of Search 74/58, 5.41, 5.42, 5.47; 73/504; 33/226 [56]References Cited UNITED STATES PATENTS 3,267,745 8/1966 Smead et a].74/5.41X 3,279,086 l0/l966 Schlitt et al 74/5.47X

OUTPUT 3,329,028 7/1967 Schafier 74/541 Primary ExaminerManuel A.Antonakas Attorney-Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: Avertical reference system is provided for supplying output electricsignals proportional to the sines of the angles of tilt of a structurewith respect to the vertical about two tilt axes at right angles to eachother. The specific system disclosed includes a vertical gyroscopehaving a rotor mounted in inner and outer gimbals on a gyroscope supportwhich in turn is rigidly secured to the structure subject to beingtilted. Two force balanced accelerometers are secured to the gyroscopesupport and each is positioned to sense the angle of tilt of thestructure about one of such tilt axes. The accelerometers are employedin conjunction with pickoffs and torquers on the axes of the gimbals tomaintain the spin axes of the gyroscope vertical. The output signals areobtained from the pickoffs on the gimbal axes.

VOLTAGE SUM MATlON DEMODULATOR as E CHOPPER i ACCELEROMETER r o I I I 30ACCELEROMETER ACQz-Q) a VOLTAGE 42 SUMMATION 4e OUTPUT 44 AMPLIFIERDEMODULATG? CHOPPER PATENTEU JUHZ 8 mm FIG. I

OUTPUT e2 VOLTAGE 42 s UMMATION OUTPUT DEMODULATOR slssmao SHEET 1 OF 33i smmq DEMODULATOR 38 CHOPPER AMPLIFIER CHOPPER JAMES W. DEER//VI/E/V70/? B) BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN AT TOR/V5 Y5PATENTEnfJunzs Ian 3, 587, 330

sum 3 OF 3 PVERTICAL \f VERTICAL BUCKHOR/V, BLORE, KLAROU/ST 8 SPAR/(MANAT T ORA/E KS VERTICAL REFERENCE SYSTEM BACKGROUND OF THE INVENTIONVertical reference systems for supplying signals proportional to thesines of the angles of tilt of a structure with respect to the verticalabout two axes at right angles to each other have been previouslyprovided in which force balanced accelerometers or other devices forsensing angles with respect to the vertical are employed in conjunctionwith a vertical gyroscope. It has heretofore been considered necessaryto mount such angle sensing devices on the inner gimbal of the gyroscopein which case the angles sensed by such devices are the angles betweenthe spin axis of the gyroscope and the vertical about two axes at rightangles to each other and at right angles to the spin axis of thegyroscope.

Amplified signals from the angle sensing devices are supplied totorquers on the correct gimbal axes to cause the gyroscope to processback to a position in which its spin axis is vertical. The signalsproportional to the sines of the angles of tilt of the structure areobtained from angle pickoffs on the gimbal axes. These tilt angles arethus the angles about the gimbal axes between the vertical and a lineassociated with the gyroscope support which is vertical when thissupport has zero tilt.

The prior art systems just described requires slip rings on the gimbalaxes to enable signals from the force balanced accelerometers on theinner gimbal to be delivered to external amplifiers. These slip ringsare in addition to the slip rings required for delivering signals fromthe pickoffs on the gimbal axes to external amplifiers and from suchamplifiers to the torquers on such axes and also similar slip rings forsupplying exciting power to such pickoffs and torquers as well as to thedriving motor for the gyroscope rotor. The slip ring structures of theprior art are thusquite complicated and this results in undesiredfrictional torques about the gimbal axes. Also the accelerometers on theinner gimbal are commensurate in size and weight with the inner gimbalitself. This lowers the nutation frequency of the gyroscope to a valuewhich can cause the gyroscope to go into sustained nutation. Also theincreased weight can cause severe mechanical vibration problems,particularly at one of the gimbal resonant frequencies. This is possiblesince necessary clearance in the gimbal bearings provides for relativemovement between the gimbals and their supports.

in accordance with the present invention the vertical sensing devicesare removed from the inner gimbal of the gyroscope and are strapped downto the structure which is subjected to tilting, for example, a vehiclewhich is driven over rough terrain. The strapping down is preferablyaccomplished by rigidly securing the frame portions of the verticalsensing devices to the gyroscope support and rigidly securing suchsupport to the structure but it is apparent that the sensing devices maybe secured directly to the structure or otherwise secured in fixedposition relative to the support. Positioning the vertical sensingdevices so as to be tilted with such structure rather than on the innergimbal has, in a specific example, enabled the elimination of four sliprings from the inner gimbal axis and five slip rings from the outergimbal axis. The nutation and vibration problems referred to above arereduced to tolerable levels or entirely eliminated.

it is therefore an object of the invention to provide a simplified andmore accurate vertical reference system.

A specific example of a vertical reference system in accordance with theinvention is shown in the attached drawing of which:

FIG. l is a diagrammatic isometric view of the mechanical features ofthe system with electric circuits indicated in block diagram thereon;

FIG. 2 is a schematic diagram of an electric circuit for producing anoutput signal proportional to the sine of the angle of tilt of thegyroscope support or structure to which it is secured about one tiltaxis with certain mechanical structure shown diagrammatically thereon;

FIG. 3 is a diagrammatic view useful in explaining the operation of thedisclosed system;

MG. 41 is a view similar to FIG. 3 showing one element of the device ina different position; and

HG. 5 is a graph also useful in explaining the operation of the system.

Referring to H6. 1 of the drawing, a vertical gyroscope 10 isdiagrammatically shown as having a. rotor mounted in inner and outergimbals l2 and lll, respectively, on a gyroscope support 16 which itwill be understood would actually be a gyroscope case. This supportwould be in turn mounted upon and form part of a structure, such as avehicle or aircraft, or the like, subject to being tilted.

Assuming that the upper surface of the support is a plane which ishorizontal when the structure referred to is not tilted, a perpendicularto such surface for example the dash-dot line 117 in MG. 1 will be at anangle to the vertical, when the structure is tilted. This angle can beresolved into two tilt angles with respect to the vertical one about theaxis indicated by the dot-dash line l8, and the other about the axisindicated by the dot-dash line 19. For convenience the axis 19 may beparallel to the outer gimbal axis 20 and the axis lib may be normal tothe axis 19 and to the outer gimbal axis. In FIG. l the axis 18 is inthe same plane as the inner gimbal axis 2t and parallel to such axis 21when the outer gimbal is in the position with respect to the base 16shown in FIG. 1.

The inner gimbal 112 is provided with an angle sensing pickoff and alsoa torquer 24 on its gimbal axis 22, A force balanced accelerometer 30 issecured. to the gyroscope support H6 so as to sense any correspondingangle between the perpendicular 117 to the upper surface of the support16 and the vertical about the axis 18. Similarly the outer gimbal M isprovided with an angle sensing pickoff 26 and a torquer 28 on its gimbalaxis 20, and a similar force balanced accelerometer 32 is secured tosuch support to sense any corresponding angle between such perpendicularand the vertical about the axis l9.

The pickoff 22 senses the angle between the perpendicular to the uppersurface of the support 16 and the spin axis of the gyroscope, indicatedby the dash-dot line 33, about the axis 21. When the spin axis 33 isvertical, the angle sensed by the pickoff 22 is equal to and opposite indirection to the angle sensed by the accelerometer 30. Thus thealgebraic sum of these angles is zero and any such sum which is not zeroresults from the spin axis of the gyroscope being out of alignment withthe vertical. An amplified combined signal which is a function of thealgebraic sum referred to is provided by a circuit including ademodulator 34 for a signal from the pickoff 22, a voltage summationcircuit 36 to which the signal from the accelerometer 30 is alsosupplied, a. chopper 38 and a high gain servoamplifier 40. Thisamplified signal from the amplifier 40 is supplied to the torquer 28 onthe outer gimbal axis 20 to produce a torque about this axis in adirection to cause the gyroscope to precess about the axis 2ll to reduceto zero the angle about this axis between the spin axis 33 and thevertical, thus reducing the amplified signal to zero.

Similarly an amplified combined signal, which is a function of thealgebraic sum of the angle sensed by the pickoff 26 on the outer gimbalaxis 20 and the angle with the vertical sensed by the accelerometer 32,is provided by the circuit including the demodulator 42, voltagesummation circuit 454, chopper 46 and high gain servoamplifier 48. Thisamplified signal is supplied to the torquer 24 on the inner gimbal axis21 to cause this torquer to apply torque about the axis 21 in adirection to cause the gyroscope to precess about the axis 20 of theouter gimbal to reduce to zero the angle about this axis between thespin axis 33 and the vertical. These actions maintain the spin axis ofthe gyroscope vertical. Alternating current output signals which are afunction of the angles of tilt of the gyroscope support to and thestructure to which it is secured with respect to the spin axis of thegyroscope about the respective gimbal axes can be obtained from thepickoffs 22 and 26, or similar direct current output signals can beobtained from the output terminals of the demodulators 34 and 42 as willbecome apparent below.

A schematic diagram of the circuits indicated on FIG. l is shown in FIG.2. This circuit may be that associated with the pickoff 26 on the axis20 of the outer gimbal and the accelerometer 32, and which includes thedemodulator 42, voltage summation circuit 44, chopper 46 and amplifier48, supplying an amplified difference signal to the torquer 24 on theaxis of the inner gimbal.

Thepickoff 26 is shown as a resolver having an exciting winding 50supplied from a single phase alternating current line 52 and positionedon one member of the resolver. Such resolver usually has two separateoutput windings on another member relatively rotatable with respect tothe first mentioned member. One winding 53 only of these two windings isemployed and is connected to the primary winding d of a transformer 55forming part of the demodulator 42.

The secondary winding 56 of the transformer 55 is center tapped and hasits end terminals connected through diodes 58 and 60 to the endterminals, respectively, of a center tapped resistor 62 havingcapacitors 6d and 66 connected across its respective halves. Thesecondary winding of a transformer 68, having its primary windingconnected across the line 52, is connected between the center tap of thewinding 56 and the center tap of the resistor 62.

It will be apparent that the demodulator 42 includes two parallel singlephase rectifier circuits supplied from the transformer 6%, whichseparately charge the capacitors 64 and 66 so as to maintain directcurrent flow in opposite directions through the opposite halves of theresistor 62. In the absence of any signal from the winding 56 of thepickoff 26 and with the demodulator circuit properly balanced, equaldirect current voltages with respect to the center tap of the resistor62 appear at the end terminals of this resistor so that there is nooutput voltage from an output terminal 74) with respect to ground.

In the position of the resolver winding 53 shown in FIG. 2, no voltageis supplied to the primary winding 54 of the transformer 55 but anyrelative rotation of the winding 53 with respect to the exciting winding50 of the resolver from the position shown in FIG. l due to relativerotation of the outer gimbal M of FIG. 1 and the gyroscope support 16about the axis of the outer gimbal due to tilting of the gyroscopesupport 16, will result in an angular relation of these windings whichwill supply a voltage to the primary winding 54. Current flow in suchwinding will result in inducing an alternating voltage in the secondarywinding 56 which is in phase with and adds to the voltage from thetransformer 68 across one-half of the winding 56 and which is l80 out ofphase with and subtracts from such voltage from the transformer 68across the other half of the winding 56. The result is that a directcurrent voltage e measured with respect to ground will appear at theoutput terminal 70 of the demodulator &2 which is proportional to thesine of the angle of rotation of the winding 52 relative to the winding50 from the position shown, and which changes in polarity in accordancewith the direction of such angle from the zero voltage position of thewinding 53. This is true since the excitation from the transformer 68supplies the iron losses of the transformer 55 and insures that thediodes are conducting and operate on the linear portions of theircharacteristics. The output from the terminal 74) of the demodulator isconnected to one terminal of a resistor 72 forming part of the voltagesummation circuit 44.

The type of force balanced accelerometer 32 shown in FIG. 2 iscommercially available and includes a pendulous mass '74 carried by oneend of an arm 76 pivoted to the frame of the accelerometer at 78. Thepivot has a core 80 secured thereto and positioned between the poles 82and 84 of a permanent magnet structure. A position sensor as issupported adjacent the mass 74. In the type of accelerometer disclosed,this sensor contains the tank circuit inductor of an oscillator circuit(not shown). The mass 743 is of conducting metal and the amplitude ofthe output of the oscillator referred to depends upon the proximity ofthe mass '74 to the sensor 36, since eddy current losses in the massload the oscillator to an extent depending upon the distance between themass and the sensor.

The closer the mass to the sensor the less the amplitude of the outputand vice versa. This is sometimes referred to as control of theamplitude of the output of an oscillator by spoiler action.

The output of the sensor 36 is delivered to a high gain amplifier 88which converts this output of the oscillator to a direct current signalwhich is zero when the mass 74 is in a predetermined position relativeto the sensor 86, and is of one polarity when the mass moves closer tothe sensor and of the other polarity when the mass moves away from thesensor. The output of the amplifier 88 is supplied to a coil 90 woundupon the core and connected in series with a resistor 92, the circuitbeing completed through ground.

Current flow through the coil is in a direction producing a torquetending to return the mass 7d to the predetermined position referred toabove with respect to the position sensor 86. The result is that thedirect current through the resistor 2 and therefore the voltage e at theterminal 94 measured with respect to ground is proportional to the sineof the angle of rotation of the entire accelerometer 32 about the axisof the pivot 7'8, the axis of this pivot being parallel to the axis 19of FIG. I.

The voltage e, from an output terminal 94 is delivered to one terminalof the resistor 96 also forming part of the voltage summation circuit44. This circuit also includes a resistor 98 which is connected betweenground and the terminals of the resistors 92 and 96 remote from theoutput terminals 70 and Q94, respectively, of the demodulator $2 andaccelerometer 32. The resistors 72 and 96 have large values ofresistance compared to the resistance of the resistor 98.

The output voltages e, and e of the demodulator 42 and the accelerometer32, respectively, are such that, if e is positive for an angle ofrotation of the outer gimbal with respect to the base 16 about the axis20, then e is positive for an angle of rotation or tilt of the base 16in the same direction about the axis 19, the latter angle being theangle the line 17 makes with the vertical. Under these conditions, thevoltage with respect to ground at the output terminal I00 of the voltagecircuit 4 3 is a direct current summation voltage equal to K(e where Kis a constant.

This summation voltage is delivered to the chopper d6 having a movablecontact 102 driven synchronously from the line 52 by a coil MM connectedacross this line. The chopped voltage is delivered to the high gainservoamplifier 48, the output of which is an alternating current signalhaving a voltage in phase or out of phase with the voltage across theline 52 and an amplitude equal to A(e,+e where A is another constant.This signal is delivered to a stator winding 108 of the torquer 24,which is of the induction motor type having a rotor lib fixed to theshaft of the inner gimbal E2 of FIG. i to apply torque to such gimbalabout its axis 18 whenever (ed-e does not equal zero. The torquer 24also has a quadrature stator winding 1112 excited through a capacitor 1Mfrom the line 52 so that such torque is developed by the torquerwhenever a voltage (e r-e exists and this torque is in the properdirection to cause the gyroscope to precess about the outer gimbal axis20 until e,+e. .=0.

The circuit including the pickoff 22, accelerometer 3J0, demodulator 34,voltage summation circuit 36, chopper 38, amplifier 40 and torquer 28may be exactly similar to the circuit described above with reference toFIG. 2, and will not be further described.

In the diagrammatic view of FIG. 3, the base 16 of the gyroscope isshown tilted about an angle BA. This is the angle of rotation of thebase about the axis 19 referred to above. Thus e is proportional to sin(BA). The outer gimbal M is shown in a position such that the spin axis33 of the gyroscope is in a vertical plane through the axis 20 and makesan angle SY with the perpendicular I17 to the support 116. This is theangle of rotation of the outer gimbal with respect to the base about theaxis 2@, also referred to above, so that the voltage 2, is proportionalto sin (SY).

If the voltage 2 is made equal to the voltage e when the angle BA equalsthe angle SY, and the resistors 99 and 72 of FIG. 2 have the sameresistance values, then K(e +e,) sin(BA) sin (sir) where K is aconstant. It is apparent that K(e +e,)=0 when BA-SY as is the case underthe conditions shown in FIG. 3.

The diagrammatic view of FIG. 4 is useful in showing that the voltageI((e +e has the necessary properties to return the spin axis 33 to avertical position, if-it is tilted from the vertical about the axis 20.This view shows the spin axis 33 tilted through an angle SP from thevertical about the axis 20. It is apparent that the angle SY of rotationof the spin axis 33 about the axis with respect to the perpendicular 17to the gyroscope base is such that S Y=SP-BA and l((e +e sin(BA)sin(SP-BA) The requirement that the voltage K(e,+e have the properproperties to be employed to return the spin axis 33 to the vertical isthat sin(BA) sin(SP-BA) be positive when the angle SP is positive and benegative when the angle SP is negative. The actual value of thisexpression is not of importance since the amplifiers 310 and 48 of FIGS.1 and 3 are usually of the type which will saturate at very small valuesoftheir input voltage K(e +e In any event, l((e,+e has a value of zerowhen SP=O.

In FIG. 5, curves of the values of sin(BA) sin(SP-BA) have been plottedagainst values of SP from 60 to +60", for various values of BA rangingfrom -60 to +60. The curves for all values of BA between -60 and +60fall within the area bounded by the curve for +60 and approximately thecurve for BA=-20 for negative values of SP and within the area boundedby the curve for BA=60 and approximately the curve for BA=+20 forpositive values of SP so that the above requirement is met for allvalues of SP and BA between 60 and +60.

It is to be noted that the curve for BA= 60 changes sign when si =+s0.This means that the angle SY=(SPBA) between the perpendicular 17 to thegyroscope base and the spin axis 30 of the gyroscope is actually l20.Similarly the angle SY for SP-60 and BA=+60 is -l20. Thus if gimbalstops are provided so that the angle SY cannot reach 1120", anypossibility of a reversal of sign of the value of sin(BA) sin(SP-BAisavoided for values of BA between :60". For other reasons, includingsocalled gimbal locking, when one of the gimbals rotates 90 with respectto the other from the position shown in FIG. l, it is desirable as apractical matter to limit the possible values of the angle SY tosomewhat less than 90. Under these conditions, for a value of Sp of+60", the value of BA cannot be more negative than -30 or for a value ofSp==-60 cannot be more positive than +30". With the restriction ofrotation of the outer gimbal to an angle SY not greater than i90, it canbe shown that the tilt of the base 16 with respect to the vertical aboutthe axis 19 (angle BA) can also approach 290 while still satisfying therequirement given above.

The same requirement applies to tilting of the base through an angle BAabout the axis 118 of FIG. 1 and the rotation the inner 2+through anangle SP about the axis 21.

It is to be noted that the amplifiers 40 and 48 are usually of a typewhich will saturate so that the actual value of K(e +e,) is not ofimportance so long as this value goes to zero when SP=0 so that SY=-BA.While the theoretical limits are thus a tilt of the gyroscope base 16through an angle just slightly less than 90 in either direction abouteither axis 18 or 19, and also angles just less than 90 in eitherdirection between the spin axis of the gyroscope and the base about theaxes 20 and 21, most requirements of use allow these angles to berestricted to angles of the order of 60.

While force balanced accelerometers of the type in which the amplitudeof the output of an oscillator circuit is controlled by eddy currentlosses in a pendulous mass mounted for movement relative to a positionsensor are shown and described in the present application, other typesof force balanced accelerometers are available depending, for example,for operation upon changes in capacitance or inductance produced by suchrelative movement. Also other types of accelerometers, for example,accelerometers providing signals directly proportional to the angle oftilt with respect to the vertical can be secured to the gyroscopesupport 16 or the structure upon which it is mounted and employed inconjunction with pickoff devices on the gimbal axis providing the sametype of signals. Thus in general, any vertical sensing device which onthe average will provide a signal which is a function of theinstantaneous angle of tilt of a structure, may be employed in thepresent system. Also in general these devices cannot be employed aloneto provide a vertical reference as they are subject to large short termerrors due to any vibration of the structure and other movementsinvolving short term accelerations. The present system, however,eliminates these errors and provides electrical signals which arefunctions ofthe instantaneous tilt of'the support.

Although it was convenient to assume for purposes of explanation of thecircuit of FIG. 3 that the voltages a and 2 are equal when the angles BAand SY are equal, this is an unnecessary restriction since exactly thesame results are obtained when the ratio of the voltage 2 to theresistance R, of resistor 72 is equal to the ratio of the voltage e tothe resistance R of resistor 96 such that e; a .lta

when BA=SY.

It is also apparent that direct current torquers can be employed so thatthe choppers 38 and 46 can be omitted or that the various electricsignals discussed above can be converted to digital signals at any placein the system and subsequent modifications of signals performed bydigital or computer operations to supply the various torques or desiredoutputs.

Iclaim:

l. A vertical reference system comprising:

a support;

a gyroscope including a rotor mounted in inner and outer gimbals on saidsupport for angular movement about innner and outer gimbal axes;

first and second pickoff means on said inner and outer gimbal axesrespectively for supplying first and second electric signalsrespectively which are functions of any angles of tilt of the supportwith respect to the spin axis of said gyroscope about said inner andouter gimbal axes respectively;

first and second sensing means for supplying third and fourth electricsignals respectively which are on the average functions ofcorresponding; angles of tilt of said support with respect to thevertical about an axis normal to the outer gimbal axis and an axisparallel to the outer gimbal axis respectively;

circuit means connected to said pick-offs and said sensing means forobtaining a first combined electric signal which is a function of thesum of said first and third signals and a second combined electricsignal which is a function of the sum of said second and fourth signals;

first torquer means for applying torque about said inner gimbal axisbetween said inner gimbal and said outer gimbal and second torquer meansfor applying torque about said outer gimbal axis between said outergimbal and said base; and

circuit means for supplying said first combined signal to said secondtorquer means and said second combined signal to said first torquermeans to thereby apply torque to said gimbals causing said gyroscope top'recess in a direction which will align said spin axis with thevertical.

2. A vertical reference system in accordance with claim 1 in which saidfirst and second pickoff means supply first and second electric signalswhich are proportional to the sines of any angles of tilt of the supportwith respect to said spin axis about said inner and outer gimbal axesrespectively and said sensing means supply third and fourth electricsignals which on the average are proportional to the sines of saidcorresponding angles of tilt of said support with respect to thevertical.

3. A vertical reference system in accordance with claim 1 in which saidsensing means are angle sensing devices secured in fixed positionrelative to said support.

4. A vertical reference system in accordance with claim 2 in which saidsensing means are each a force balanced accelerometer having a framerigidly secured with respect to said support.

5. A vertical reference in accordance with claim 1 in which each saidpickoffs supplies an alternating electric signal and each said anglesensing means supplies a direct current signal,

said circuit has means for converting said alternating electric signalsto separate direct current signals prior to obtaining said combinedelectric signals so that said combined electric signals are directcurrent amplified electric signals, and

said circuit also contains means for converting said direct currentamplified electric signals to alternating current signals prior tosupplying them to said torquers.

CERTIFICATE OF CORRECTION Patent No. 3, 587,330 Dated June 28, 1971Inventor (s) James W. Deer It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 18, "process" should read --precess-.

Column 5, line 49, "sin (SP-BA" should read --sin (SP-PEA) with respectto the value of SP--;

Column 5, line 64, 'after "rotation" insert --of--;

Column 5, line 65, change "2+" to --gimbal--.

Signed and sealed this lhth day or December 1971.

(SEAL) Attest:

ROBERT GO'ITSCHALK Attesting Officer Patents

